gcc-source: Drop gengtype manipulation

Whilst we patch gengtype.cc, we don't patch gengtype-lex.cc which would
be the file which would trigger regeneration of files.

The real bug that was likely the cause for this fix is probably SDE issues
with gcc shared workdir so this code can now be dropped.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>