ext4: delete some unused tracepoint definitions

ext4: delete some unused tracepoint definitions

A number of tracepoint instances have been removed from ext4 by past
patches but the definitions of those tracepoints have not.

All instances of ext4_ext_in_cache and ext4_ext_put_in_cache were
removed by commit f1d6bae64c47 ("ext4: remove single extent cache").
ext4_get_reserved_cluster_alloc was removed by commit 35c80bbdb6b5
("ext4: reduce reserved cluster count by number of allocated
clusters").  ext4_find_delalloc_range was removed by commit
33b058df6ed0 ("ext4: reimplement ext4_find_delay_alloc_range on extent
status tree").

All instances of ext4_direct_IO_enter and ext4_direct_IO_exit were
removed by commit 0cfa92af90d7 ("ext4: introduce direct I/O write
using iomap infrastructure").

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20210216191634.20957-1-enwlinux@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>