Following on from the previous commit, this commit removes yet more
'invalid=True' lines from gdbarch_components.py where the invalid
setting has no effect.

Due to the algorithm used in gdbarch.py for generated verify_gdbarch,
if a component has a postdefault value then no invalid check will ever
be generated for the component, as such setting 'invalid=True' on the
component is pointless.  This commit removes the setting of invalid.

There is no change in the generated code after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>