gammu: Remove python dependency, fix lib symlinks

gammu: Remove python dependency, fix lib symlinks

This removes the python dependency from the package; according to the
maintainer[1][2] the software does not depend on python.

This also fixes the symlinks for libGammu.so and libgsmsd.so.
Previously, the symlinks were overwritten by $(INSTALL_BIN) with copies
of their sources.

[1]: https://github.com/openwrt/packages/issues/8893#issuecomment-539136531
[2]: https://github.com/openwrt/packages/issues/8893#issuecomment-539152794

Signed-off-by: Jeffery To <jeffery.to@gmail.com>