Fix static analyzer warnings

runtime/base/variant_map.h: Potential leak of memory pointed to by field
'first' + 'second'

The static analyzer is smart enough to figure out that key.Clone()
allocates memory, but can't figure out that Remove(key) will remove the
key from the map. For more, please see the discussion on patch set 1 of
variant_map.h in https://android-review.googlesource.com/#/c/416101 .

Bug: 32619234
Test: test-art-host. Rebuilt ART with the analyzer to verify that these
issues are gone.

Change-Id: I8ebec5dc0d1f8863b316515d7d16a1e1a625132d