[Coverage] Remove two overloads of CoverageMapping::load. NFC.

These overloads are essentially dead, and pose a maintenance cost
without adding any benefit. This is coming up now because I'd like to
experiment with changing the way we store coverage mapping data, and
would rather not have to fix up the old overloads while doing so.

Testing: check-{llvm,profile}, build clang.
llvm-svn: 306776