Followup to the original patch for the bug.

The two functions int10_to_str() and longlong10_to_str() are identical
on many 64-bit platforms (those where both long int and long long int
are 64 bits wide). On other platforms, longlong10_to_str() could
replace int10_to_str().

Microbenchmarks suggest that having a 32-bit variant of these
functions does not have any significant benefit on 64-bit hardware, so
there is little value in having both functions. This patch removes
int10_to_str().

Change-Id: I1fec705e37f07bf5ec2fdfe8d29e17123ba52030