gcc-final: Drop obsolete code for handling lib64

Since 5628776c
"toolchain-external: fix lib64 symlinks"

lib64 is a symlink to lib so there is no reason to copy the libraries
to lib64 anymore. This fixes multiple building problems
for buildroot toolchains:

[...]
mv: 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib64/libuClibc-0.9.33.2.so'
and 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib/libuClibc-0.9.33.2.so'
are the same file
mv: 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib64/libutil-0.9.33.2.so'
and 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib/libutil-0.9.33.2.so'
are the same file
mv: 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib64/libutil.so.0'
and 'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib/libutil.so.0'
are the same file
rmdir: failed to remove
'output/host/usr/mips64-buildroot-linux-uclibc/sysroot/lib64': Not a directory
[...]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>