xfs: Remove two dead transaction log reservaion macros

Upstream commit 5b292ae3
	xfs: make use of xfs_calc_buf_res() in xfs_trans.c

Beginning from above commit, neither XFS_ALLOCFREE_LOG_RES() nor
XFS_DIROP_LOG_RES() is used by those routines for calculating
transaction space reservations, so it's safe to remove them now.

Also, with a slightly update for the relevant comments to reflect
the ideas of why those log count numbers should be.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>