usb: gadget: storage_common: Drop #ifdefs used for the sake of FSG.

storage_common.c has been used by both file_storage.c and f_mass_storage.c
which had some different requirements in a few places.  To accomodate for
that, storage_common.c provided configuratian macros which were to be
defined (or not) prior to the file #inclusion.  Because now
file_storage.c is no longer with us, we can remove support for those
macros and thus simplify the code slightly.

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