[ELF] Remove .strtab deduplication

D118577: the 0.1~1.1% .strtab size reduction does not justify the 3~6%
link time increase. Just remove it even for -O2. release/14.x
has D118577 and the release note mentioned that this may be removed.

Fix https://github.com/ClangBuiltLinux/linux/issues/1578
caused by D118577 (empty string not in stringMap).