Make code warnings-free with -pedantic, this should guard against more programming errors in the future

This removes a function that's no longer used, and inline specifiers
from include file (where they're useless anyway). This makes the whole
default project warnings-free with -Wall -pedantic.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
A call to that function was removed in this commit:

https://github.com/multiwii/baseflight/commit/a753661c23c428ede8c2b72e95d3e31f6c8b894e