cmake: Don't test whether the installed west is the bootstrapper

Of the 1.7s or so that CMake spends in Configure-time there is 0.1s
spent to determine whether west can flash or not. This is significant
compared to the usefulness of flash-check.

AFAICT when installing west there is no longer a two-step procedure
where first a bootstrapper that can't flash is installed, and then a
proper west is installed, so the risk of having installed the
bootstrapper is no longer something we should spend 100ms testing for.

This patch removes the check, thereby speeding up CMake by 100ms.

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