Remove the shadowed file-decoration-style in gruvmax-fang

[VXNCXNX]

Fixes #2211.

What's broken

gruvmax-fang sets file-decoration-style twice, on consecutive lines:

file-decoration-style = "#434C5E" ul
file-decoration-style = "#84786A" ul

git takes the last value, so the first line has never had any effect. It looks like a copy-paste from nord, which uses that same #434C5E at line 336.

It is not purely cosmetic: a strict INI reader refuses the whole file. Python's configparser raises DuplicateOptionError on it, which is what the issue reports.

The fix

Delete the shadowed line. Nothing else in the file has a duplicate key, so this is the only one.

Verification

Effective value is unchanged:

$ git config -f themes.gitconfig --get 'delta.gruvmax-fang.file-decoration-style'
#84786A ul     # identical before and after

And the file now parses under a strict reader:

configparser.ConfigParser(strict=True).read("themes.gitconfig")   # 21 sections, no error