mtd: imagetag: remove unused imagestart variable

mtd: imagetag: remove unused imagestart variable

The imagestart variable was assigned sizeof(tag) (which incorrectly
computed the pointer size rather than the struct size), but was never
actually read after the assignment in either trx_fixup() or
mtd_fixtrx(). The CRC calculations that follow use
offsetof(struct bcm_tag, header_crc) directly.

Remove the dead variable and its assignments entirely rather than fixing
the sizeof expression. No functional change.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23546
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>