rm "I/O error" error msg; just keep >0 exit status (re: 9011fa93)

The bug was really that I/O errors in output builtins were
undetectable by any means. Having a >0 exit status is sufficient.
Adding an error message risks making existing ksh scripts noisier,
or even breaking them if they redirect stderr to stdout.

Note to self: in future, implement the minimum change necessary to
fix bugs, nothing more. The fact that I needed to add four extra
2>/dev/null to the regression tests should have been a hint.

src/cmd/ksh93/bltins/print.c,
src/cmd/ksh93/data/msg.c,
src/cmd/ksh93/include/io.h:
- Remove "I/O error" message.

src/cmd/ksh93/tests/builtins.sh:
- Update to check for exit status only.

src/cmd/ksh93/tests/basic.sh,
src/cmd/ksh93/tests/coprocess.sh:
- Revert four new '2>/dev/null' to suppress the error message.

(cherry picked from commit 5e17be24d18455b575b6e98bc631c6935ffc795a)

src/cmd/ksh93/tests/builtins.sh

@@ -662,8 +662,8 @@ actual=$(
         ) | true
     } 2>&1
 )
-expect=$': print: I/O error\n1'
-if [[ $actual != *"$expect" ]]
+expect='1'
+if [[ $actual != "$expect" ]]
 then
     err_exit "I/O error not detected: expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 fi