Fix a warning about redefining arraysize

Simply removing the redeclaration yields new signed/unsigned conversion
warnings, since the previously-defined arraysize (from
android-base/macros.h) hands back a size_t.

Rather than #undef'ing and continuing on, it looks like we can
trivially get away without needing arraysize at all.

Bug: None
Test: mma. Warnings disappeared.
Change-Id: I56da4dfb0e29edec10d276147e7b5d99fc32c65b