[lldb][NFC] Delete deleted const char* overloads of SetValueFromString

This came up during the review of D96817 because those deleted overloads force
the caller to explicitly create a StringRef when passing a string literal.

It seems they were added as some kind of help while migrating the code base to
StringRef in D24847, but I don't think they have any use these days and make
these functions awkward to use.

This patch just removes all the deleted overloads.

Reviewed By: tatyana-krasnukha

Differential Revision: https://reviews.llvm.org/D96861