Remove unused execution paths when formatting

fastboot uses the internal functions generate_ext4_image and
generate_f2fs_image when called via the `format` subcommand. An option
for populating the newly created fs with an initial directory exists,
but it is not exposed to the command line interface (initial_dir is
always ""). Remove the unused codepaths for this preloading.

Test: atest --host fastboot_test
Bug: 237960487
Change-Id: I7acfe6352cf26b5cbe0e5553b288c3798e96a893