Revert "[clangd] Enable expand-auto for decltype(auto)."

This reverts commit 37ec65e1.

Its prerequisite 55d96ac3 wsa
reverted in c2293bc1. c2293bc1's
patch description claimed that it reverted 37ec65 as well,
but it apparently didn't.

See https://reviews.llvm.org/D116921#3231802