extension: remove extra kde-neon build envvars

[mr-cal]

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• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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Cherry-pick #4193 into hotfix/7.5.

[sergiusens]

@ScarlettGatelyMoore are you sure this change is safe?

[codecov-commenter]

Codecov Report

Merging #4242 (eea33f2) into hotfix/7.5 (b6bd029) will increase coverage by 0.00%.
The diff coverage is n/a.

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@@             Coverage Diff             @@
##           hotfix/7.5    #4242   +/-   ##
===========================================
  Coverage       94.84%   94.85%           
===========================================
  Files             649      649           
  Lines           54346    54379   +33     
===========================================
+ Hits            51545    51580   +35     
+ Misses           2801     2799    -2     

Impacted Files	Coverage Δ
snapcraft/extensions/kde_neon.py	100.00% <ø> (ø)
tests/unit/extensions/test_kde_neon.py	100.00% <ø> (ø)

... and 18 files with indirect coverage changes

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[ScarlettGatelyMoore]

@ScarlettGatelyMoore are you sure this change is safe?

Yes. After removing these two, all my random build failures started building, also other reports on the forums reported this fix fixed their issues. Builds that worked before still work.

[sergiusens]

thanks for getting back to me @ScarlettGatelyMoore
We shall move forward with it, if you can, please keep an eye on any errors people report on the forum about this extension