MIPS: Drop unused PTR_COUNT_SHIFT from u-boot.lds

The u-boot.lds linker script for MIPS defines a PTR_COUNT_SHIFT macro to
2 or 3 for 32 bit or 64 bit builds respectively. This macro is never
actually used though, so remove the dead code.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>