usb: gadget: storage_common: remove FSG_BUFFHD_STATIC_BUFFER support

Since f_mass_storage stopped using FSG_BUFFHD_STATIC_BUFFER (because it
caused buffers not to be page aligned which did not work well with at
least some UDCs), no code was using it.  Removing not to bloat the code
too much.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>