cmake: Remove dead code 'FindRegex.cmake'

When Zephyr was building the C implementation of KConfig it used the
build script 'FindRegex.cmake' to satisfy a regex dependency.

Now that we no longer use the C implementation of Kconfig we no longer
use this build script, and it is safe to remove it.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>