Remove legacy rc.d infrastructure references from rc(8)

Legacy rc.d scripts (.sh extension) have not been supported since
r193118. Remove the outdated references to the legacy format, as they
are no longer valid.

Bug:	193936
MFC after:	1 week
Reviewed by:	cress, emaste (mentor)
Differential Revision:	https://reviews.freebsd.org/D18666

share/man/man8/rc.8

@@ -31,7 +31,7 @@
 .\"     @(#)rc.8	8.2 (Berkeley) 12/11/93
 .\" $FreeBSD$
 .\"
-.Dd September 18, 2018
+.Dd December 29, 2018
 .Dt RC 8
 .Os
 .Sh NAME
@@ -156,9 +156,6 @@ which sets
 to
 .Dq Li start ,
 and sources the script in a subshell.
-If the script has a
-.Pa .sh
-suffix then it is sourced directly into the current shell.
 Stop processing when the script that is the value of the
 .Va $early_late_divider
 has been run.
@@ -219,9 +216,6 @@ which sets
 to
 .Dq Li stop ,
 and sources the script in a subshell.
-If the script has a
-.Pa .sh
-suffix then it is sourced directly into the current shell.
 .El
 .Ss Contents of Nm rc.d/
 .Nm rc.d/
@@ -261,13 +255,6 @@ as well as services which might run commands as users
 and
 .Pa sendmail ) .
 .El
-.It Pa foo.sh
-Scripts that are to be sourced into the current shell rather than a subshell
-have a
-.Pa .sh
-suffix.
-Extreme care must be taken in using this, as the startup sequence will
-terminate if the script does.
 .It Pa bar
 Scripts that are sourced in a subshell.
 The boot does not stop if such a script terminates with a non-zero status,