Since generated names now have their package names prefixed, the
extra prefix, "Go", is both confusing and counter-productive to
making the generated ObjC code look like any other native code.
Change the default to the empty prefix, while preserving support
for an explicit prefix if needed.
This is a backwards incompatible change; to keep the old behaviour,
specify "-prefix Go" to the gobind or gomobile command.
While we're here, fix the Ivy example for the recent change in
error returns.
Change-Id: I7fef4a92a18ddadee972ccf359652e3b31624f33
Reviewed-on: https://go-review.googlesource.com/34643
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
