tile: Remove undefined API
shumate_tile_get_modified_time, shumate_tile_set_modified_time, shumate_tile_get_etag, and shumate_tile_set_etag where removed in commit 7a80cf63 The leftover in the header cause the Gir file to incorrectly have these Causing weird link errors on Rust.
Signed-off-by: Hubert Figuière hub@figuiere.net