Remove invisible variation selector-15 from keymap_japanese.h

[precondition]

Description

Some U+2194 LEFT RIGHT ARROW ↔ symbols in keymap_japanese.h were, for some reason, combined with the invisible marking, non-spacing character U+FE0E VARIATION-SELECTOR-15. Naturally, because this PR removes an invisible combining character, you won't be able to see anything in the diff.

~/qmk_firmware/quantum/keymap_extras on develop ⇣2 *6 ⨤9                                                                                                                                              
❯ echo "import unicodedata; print(unicodedata.name('\ufe0e'))" | python3
VARIATION SELECTOR-15

~/qmk_firmware/quantum/keymap_extras on develop ⇣2 *6 ⨤9                                                                                                                                              
❯ rg "\ufe0e"
keymap_japanese.h
31: * │Z↔︎H│ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 0 │ - │ ^ │ ¥ │   │
43:#define JP_ZKHK KC_GRV  // Zenkaku ↔︎ Hankaku ↔ Kanji (半角 ↔ 全角 ↔ 漢字)

~/qmk_firmware/quantum/keymap_extras on develop ⇣2 *6 ⨤9                                                                                                                                              
❯ git checkout remove-variation-selector-15 
Switched to branch 'remove-variation-selector-15'
Your branch is up to date with 'origin/remove-variation-selector-15'.

~/qmk_firmware/quantum/keymap_extras on remove-variation-selector-15 *6 ⨤9                                                                                                                            
❯ rg "\ufe0e"                              

~/qmk_firmware/quantum/keymap_extras on remove-variation-selector-15 *6 ⨤9                                                                                                                            
❯ 

leftrightarrow.webm

Footage of keymap_japanese.h open in Vim where I press ga to display information relative to the character under the cursor.
A normal left-right arrow symbol with no combining characters prints

<↔> 8596, Hex 2194, Oct 20624, Digr <>

but one which is combined with variation-selector-15 prints:

<↔> 8596, Hex 2194, Oct 20624, Digr <> < ︎> 65038, Hex fe0e, Octal 177016

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).