Remove dead pattern on ARM.

Remove dead pattern on ARM.

About 13 years ago the reg-to-reg patterns were split up, before that time
output_move_double could actually handle this case.

After the split was done most patterns were updated except for *neon_mov
which incorrectly retained reg,reg as a valid alternative.

However output_move_double can not handle this and simply returns ""
and asserts.

This pattern is essentially dead and I'm removing it for clarity.

gcc/
2018-08-16  Tamar Christina  <tamar.christina@arm.com>

	* config/arm/neon.md (*neon_mov<mode>): Remove reg-to-reg alternative.

From-SVN: r263582