patch 9.1.0535: newline escape wrong in ex mode

Problem:  newline escape wrong in ex mode (Konrad Schwarz)
Solution: partly revert patch 7.3.014, remove backslash in front of a
          newline when not in prompt mode in ex line mode
          (Mohamed Akram)

This fixes newline escaping to allow passing multiple commands to
":global", multiple lines to shell commands, and ending lines in append
mode with backslashes. This should fix a POSIX/(traditional) VI
incompatiblity.

This reverts a previous incorrect attempt at patch v7.3.014 to fix
append mode which removed half of trailing backslashes which lead to,
eg. the following two commands being parsed as having a different number
of backslashes:

```
!echo foo\\\
```

```
!echo foo\\ \
```

fixes: #6135
fixes: #7244
closes: #15120

Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 59d3db4..1dc28bc 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2341,12 +2341,7 @@
     {
 	for (p = ea.arg; *p; ++p)
 	{
-	    // Remove one backslash before a newline, so that it's possible to
-	    // pass a newline to the shell and also a newline that is preceded
-	    // with a backslash.  This makes it impossible to end a shell
-	    // command in a backslash, but that doesn't appear useful.
-	    // Halving the number of backslashes is incompatible with previous
-	    // versions.
+	    // Remove one backslash before a newline
 	    if (*p == '\\' && p[1] == '\n')
 		STRMOVE(p, p + 1);
 	    else if (*p == '\n' && !(ea.argt & EX_EXPR_ARG))