Use libgroove.pc from source code

Drop the `libgroove.pc` file from the libgroove-git package. Use the pkg-config
file included in libgroove's source code instead of shipping an extra copy in
the AUR package. This change helps prevent any synchronization issues between
what upstream provides and what the AUR package installs.

Note that the libgroove-git package and the libgroove.pc file advertise
different versions. For example, libgroove-git and libgroove.pc may advertise
these versions, respectively:

    4.2.1.2.g4b2801b
    4.2.1

This is to done to make sure that `libgroove.pc` always advertises the same
version as the library itself.