Remove the part of test/Sema/pragma-pack-2.c that has irregular "#pragma pack"s.

Before r151307 this part worked without compiler errors but now it only verifies
that our handling of irregular pragmas is broken compared to gcc, it has no
practical usefulness; it creates invalid structs that cannot be used for 'offsetof' testing.

If we later decide to handle irregular pragmas without compiler errors we can
put back this part.

llvm-svn: 152480