fs_mgr: removing __BEGIN_DECLS and __END_DECLS

fs_mgr isn't usable by C now as we've migrated all fs_mgr clients in AOSP
to CPP. Removing the support for C clients.

Bug: 67024518
Bug: 67444051
Test: boot sailfish

Change-Id: Iac57f51eb6a524c6dcdc625c83af75602b4f9b3b