Remove check that depends on malloc implementation.

Remove check that depends on malloc implementation.

Summary:
- The EXPECT_EQ(vec.capacity(), vec2.capacity()) check is implicitly
testing how the platform's malloc() and malloc_usable_size()
implementations interact. We know that small_vector will attempt to
allocate storage for 3 elements via malloc(), but the small_vector
implementation then uses malloc_usable_size() to determine capacity,
which can return more than the size requested by malloc(). As a
result, capacity may grow more than expected depending on how the
malloc implementation works.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: alandau@fb.com

FB internal diff: D1008922