[clang-format] Remove special case for kw_operator when aligning decls

This change breaks no existing tests but does fix the linked issue.
Declarations of operator overloads are annotated with
`TT_FunctionDeclarationName` on the `operator` keyword, which is already
being checked for when aligning, so the extra `kw_operator` doesn't seem
to be necessary. (just for reference, it was added in
rG92b397fb / 92b397fb)

Fixes https://github.com/llvm/llvm-project/issues/55733

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D137223