Enable subversion support by default and remove git-subversion.

Without PROVIDES/REQUIRES support, ports must depend on exactly one git
port/pkg. Subversion is currently FreeBSD's primiary version control
system and it is widely used with git so it makes sense to enable it by
default and provide a consistent user experience.

Approved by: garga
Differential Revision: https://reviews.freebsd.org/D22496