MAINTAINERS: fix git tree url for musb module

Commit 3b2435192fe91 ("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 <[e-mail]>
Signed-off-by: Bin Liu <[e-mail]>
Acked-by: Felipe Balbi <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

MAINTAINERS

···
 MUSB MULTIPOINT HIGH SPEED DUAL-ROLE CONTROLLER
 M:	Bin Liu <[e-mail]>
 L:	[e-mail]
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/balbi/usb.git
 S:	Maintained
 F:	drivers/usb/musb/