Remove Externals/, used to checkout svn externals in the old days

d2876720-25f4-11e4-a734-c80aa9e67bbd
Remove dead symlinks that used to point to scripts under Externals/

$ git rm asvn svnbranchroot svnmimeset.sh

The scripts are still in the Git/svnutils/ submodule, but will probably
never use them. Heck, don't know if they actually work anymore as
they're written for ancient versions of svn.

59a4190a-25f6-11e4-90be-c80aa9e67bbd
Makefile: update: Remove Externals update

7aabf466-273f-11e4-95c0-c80aa9e67bbd
Merge branch 'remove-old-svn-stuff', remove Externals/

Yet more svn stuff that should be deleted.

* remove-old-svn-stuff:
  Makefile: update: Remove Externals update
  Remove dead symlinks that used to point to scripts under Externals/
  Remove Externals/, used to checkout svn externals in the old days

fa4f98f8-273f-11e4-87cb-c80aa9e67bbd