Remove EXCLUDE_FS_CONFIG_STRUCTURES

Long overdue clean up; there is no reason to include fs_config.h in
android_filesystem_config.h, and the fact that it causes problems if
it's done, necessitating EXCLUDE_FS_CONFIG_STRUCTURES to exist, is a
good reason to fix it.

Bug: 149785767
Test: build
Change-Id: I5ba2541cd1045c22486140649c90db39163081fd