[clangd] Remove redundant check for renamed symbol origin

This is a follow-up on D116643. `isInSystemHeader` check already detects
symbols coming from the Standard Library, so searching for the qualified name
in StdSymbolMap.inc is no longer necessary.

The tests filtering out purely based on the symbol qualified names are removed.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D117491