Revert "[ADT] add StringViewExtras llvm::starts_with for std::string_view"

This reverts commit d6d30dd9.

These helpers were added to be used by LLVMDemangle, which would be a
layering violation for that to depend on LLVMSupport. Undo this commit,
and I'll these these directly in LLVMDemangle.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148547