In https://github.com/ARMmbed/mbed-os-tf-m-regression-tests/pull/96
we relocate the path to clone TF-M to a shorter one. This solves
an unsupported long path issue on Windows, and also makes the TF-M
and tests easier for developers to locate. Hence there's no need
to ignore the old path.

Note: For TF-M v1.0, we don't support building them from source
anymore, so it's also been removed from .gitignore.