Update the build infrastructure to use Git instead of Subversion

Summary:
The doc repository is not mirrored to Subversion so updating the
repository from Subversion does not make sense. Remove
Subversion-specific files too.

While here remove now defunct $FreeBSD$ markers in affected files.

Test Plan:
make clean
make update
make

Reviewed by: bcr, ygy, doceng (gjb)
Differential Revision: https://reviews.freebsd.org/D27678
Subscribers: drewery, emaste