fscache: remove FSCACHE_OLD_API Kconfig option

Commit 01491a75

 ("fscache, cachefiles: Disable configuration") added
the FSCACHE_OLD_API configuration when rewritten. Now, it's not used any
more. Remove it.

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://listman.redhat.com/archives/linux-cachefs/2022-March/006647.html # v1