std.fifo.LinearFifo: remove ensureCapacity (deprecated in 0.9)

Followup to 902df103c6.
Signed-off-by: Eric Joldasov <bratishkaerik@getgoogleoff.me>

lib/std/fifo.zig

@@ -117,8 +117,6 @@ pub fn LinearFifo(
             }
         }
 
-        pub const ensureCapacity = @compileError("deprecated; call `ensureUnusedCapacity` or `ensureTotalCapacity`");
-
         /// Ensure that the buffer can fit at least `size` items
         pub fn ensureTotalCapacity(self: *Self, size: usize) !void {
             if (self.buf.len >= size) return;