Remove Residual Expat

Google originally introduced EXPAT as a dependency to gdb 7.3 here https://github.com/SaberMod/toolchain_build/commit/5ef7fa6f214e96660a9b16343fea904831e5e6e2

Now 5 versions of gdb later it seems this dependency is no longer needed (I tested this commit on aarch64, arm-eabi, and androideabi and they all built fine. The toolchains also worked fin on a ROM build I ran afterwards).

Furthermore building expat adds an additional 15 seconds or so to build time which may not seem like a lot but it adds up over time. Also keep in mind that 15 seconds is 3-5% of the build time when compiling arm-eabi toolchains so why waste our time with this? (we have better things to do)

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>