MB-7882 Binary optimize remove_block_prefixes/2

Using the +bin_opt_info flag, the Erlang compiler told the
remove_block_prefixes/2 function was not binary optimized:

/opt/r14b04/bin/erlc -I../../src +bin_opt_info  couch_file.erl;
./couch_file.erl:564: Warning: INFO: matching anything else but a plain variable to the left of binary pattern will prevent delayed sub binary optimization; SUGGEST changing argument order

After this change, the compiler now tells us it is optimized:

/opt/r14b04/bin/erlc -I../../src +bin_opt_info  couch_file.erl;
./couch_file.erl:564: Warning: OPTIMIZED: creation of sub binary delayed
./couch_file.erl:570: Warning: OPTIMIZED: creation of sub binary delayed

Change-Id: I746dcf440a83789302a801318d4839e97b7c5287
Reviewed-on: http://review.couchbase.org/25395
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>