boot: remove gptboot.efifat, it never should have been

conical hat reduction: Make sure we also remove gotboot.efifat. It was created,
briefly, and shouldn't have existed in the first place. Kill it at the same
place we kill boot1.efifat.

Pointy Hat to: imp@
Approved by: re@ (gjb)
(cherry picked from commit 8cd1b2b1a7ab55e96aaf40a401d8792f2697b274)
(cherry picked from commit dd9b24be14d4311ca6056f41a2d81801d2c0fa03)