Remove duplicate COMBINING HORN in keymap_us_extended.h

[precondition]

Description

This is another one of those PRs where you can't see any difference in the diff, sorry.

[ins] In [29]: import unicodedata as uc

[ins] In [30]: before = "// ̛̛  (dead)"

[ins] In [31]: after = "// ̛  (dead)"

[ins] In [32]: list(map(uc.name, before))
Out[32]: 
['SOLIDUS',
 'SOLIDUS',
 'SPACE',
 'COMBINING HORN',
 'COMBINING HORN',
 'SPACE',
 'SPACE',
 'LEFT PARENTHESIS',
 'LATIN SMALL LETTER D',
 'LATIN SMALL LETTER E',
 'LATIN SMALL LETTER A',
 'LATIN SMALL LETTER D',
 'RIGHT PARENTHESIS']

[ins] In [33]: list(map(uc.name, after))
Out[33]: 
['SOLIDUS',
 'SOLIDUS',
 'SPACE',
 'COMBINING HORN',
 'SPACE',
 'SPACE',
 'LEFT PARENTHESIS',
 'LATIN SMALL LETTER D',
 'LATIN SMALL LETTER E',
 'LATIN SMALL LETTER A',
 'LATIN SMALL LETTER D',
 'RIGHT PARENTHESIS']

[ins] In [34]: horn_comment_from_keymap_bepo = "// ̛  (dead)"

[ins] In [35]: list(map(uc.name, horn_comment_from_keymap_bepo))
Out[35]: 
['SOLIDUS',
 'SOLIDUS',
 'SPACE',
 'COMBINING HORN',
 'SPACE',
 'SPACE',
 'LEFT PARENTHESIS',
 'LATIN SMALL LETTER D',
 'LATIN SMALL LETTER E',
 'LATIN SMALL LETTER A',
 'LATIN SMALL LETTER D',
 'RIGHT PARENTHESIS']

I care about this invisible change because as a part of adding support for intl/locale legends to QMK configurator, I wrote a script that converts keymap_extras header files from qmk_firmware into JS objects that the QMK configurator can use to display the correct key legends. For display purposes, as is the standard, I combine diacritics with the dotted circle (◌) but if there are two combining diacritics, then two dotted circles are displayed; not desirable.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization (as in better comments)
• 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).