arm: omap2: Drop MACH_OMAP3517EVM entry

The MACH_OMAP3517EVM was added back in commit 549f95ed due to
being used by the ASoC driver, but this driver was dropped in
commit 2c2596f3 as the audio for this board had been moved to
a simple-audio-card configuration so MACH_OMAP3517EVM is now
properly unuused and can be dropped.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>