Remove DCHECK_GE checks from fixed_array.h

This triggers -Wtype-limits warnings on comparisons
which are always true, since the test being done is
n >= 0, where n is of type size_t, which is always
true.

This causes problems when compiling Ceres on linux
with miniglog.

Change-Id: Ia1d1d1483e03469c71fde029b62ca6d84e9b27e0