arm: omap2: Drop obsolete MACH_OMAP3_PANDORA entry

The MACH_OMAP3_PANDORA is no longer referenced anywhere in the kernel
options so it can now be dropped as the board has long moved to DT.

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