[AMDGPU] Fix compilation failure when IR contains comdat

Remove a useless SwitchSection which also causes compilation failure
when IR contains comdat.

The SwitchSection is useless because the current section is already
correct text section for the function therefore no need to switch.

It causes compilation failure for comdat because functions with comdat
has specific text section, not the default .text section.

Since HIP uses comdat, this bug caused failures for HIP.

Differential Revision: https://reviews.llvm.org/D46770

llvm-svn: 332137