[utils] Remove unused CMake var from LLDB build

[bulbazord]

This variable isn't used at all in LLDB's CMake build. It does appear in LLDB's XCode build for testing, so we can't remove it there.

cc @adrian-prantl @compnerd

[compnerd]

Might be nice to keep it in sync, but reducing the number of variables we are passing around is certainly desirable

[compnerd]

@swift-ci please smoke test

[weliveindetail]

Yes that's fine. (Same for LLDB_PATH_TO_CLANG_SOURCE and probably possible also for Swift, since we use find_package there too.)

[bulbazord]

Yes that's fine. (Same for LLDB_PATH_TO_CLANG_SOURCE and probably possible also for Swift, since we use find_package there too.)

Ooh, definitelyLLDB_PATH_TO_CLANG_SOURCE can be removed. Looks like there's one reference to it in the CMake but we don't do anything with it.. It does look like we do use LLDB_PATH_TO_SWIFT_SOURCE but that can probably be removed with a bit of work. I'll go ahead and remove the Clang one here. :)

[compnerd]

@swift-ci please smoke test and merge