Remove dead code.

It seems the code was meant to add the toolchain to ANDROID_BUILD_PATHS
and then export to PATH by lunch.
But now we export the toolchain to PATH explicitly in envsetup.sh and
the code is unnecessary.
This reduces the places to change when we upgrade the gcc version.
It also fixed the bug that the mips toolchain was always exported
regardless of the target arch.

Change-Id: Iee3b895b4c4e0df8971c27c01b9ecbd591848b12