MAINTAINERS: fix git tree url for musb module

Commit 3b243519 ("MAINTAINERS: drop OMAP USB and MUSB
maintainership") switched the maintainer for musb module, but didn't
update the git tree location.

Delete the git tree information, since the current maintainer doesn't
have a public tree.

Reported-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>