Remove the ARM GCC compiler extdeps to use container GCC (#388)

Removes the ARM & AARCH64 GCC compiler extdep files. Thes external
dependencies frequently break, and all Project MU builds should now be
using the container provided GCC binaries.

- [x] Impacts functionality?
- [ ] Impacts security?
- [x] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

Local build test in container.

All consumers that require the ext dep must now carry their own copies
of the extdep files removed by the commit.

[BaseTools/Bin/gcc_aarch64_linux_ext_dep.yaml](https://github.com/microsoft/mu_basecore/blob/1bd4b3bfd657ad5a3940d072be6540f7c9c23c5d/BaseTools/Bin/gcc_aarch64_linux_ext_dep.yaml)

[BaseTools/Bin/gcc_arm_linux_ext_dep.yaml](https://github.com/microsoft/mu_basecore/blob/1bd4b3bfd657ad5a3940d072be6540f7c9c23c5d/BaseTools/Bin/gcc_arm_linux_ext_dep.yaml)

(cherry picked from commit 0a8242f)