Remove unused LLVMBuild.txt files (#2282)

The llvm-build tool has been removed by upstream commit 9218ff5
("llvmbuildectomy - replace llvm-build by plain cmake", 2020-11-13),
so these files no longer serve any purpose.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@1f9a8fb

Author: svenvh