Delete .ist from TeX.gitignore (#3040)

.ist files are makeindex style files, which determine how a
makeindex-generated index will look like. Therefore, they must be
included in source control.

TeX.gitignore

@@ -144,7 +144,6 @@ acs-*.bib
 *.idx
 *.ilg
 *.ind
-*.ist
 
 # minitoc
 *.maf