Remove extern specifiers from function prototype declarations

The extern specifier in this case has no effect as the default
for functions is to have external linkage. However the extern
specifier affects code search ability to explore function
declarations and occurences as the prototype differs from
the declaration.

Test: mmm packages/modules/Bluetooth
Bug: 279964152
Change-Id: I4f2e680d49068a0f708c40373bbe865e110f7db1