Manually revert "Make COMPUTE_OFFSET and COMPUTE_OFFSET_AND_REMAINING macros."

This reverts commit b136182ad41b0552421992f36f3b70fa497fd658

This function are already marked inline, and profiler don't show much difference in performance,
revert as previous version is cleaner.

Change-Id: I1ac2c30a91b46278730ceee127efa086c7fbc6d6
Reviewed-on: https://code.wireshark.org/review/2828
Reviewed-by: Anders Broman <a.broman58@gmail.com>