patch 8.2.1172: error messages when doing "make clean" in doc or tee

Problem:    Error messages when doing "make clean" in the runtime/doc or
            src/tee directories.
Solution:   Use "rm -f".
diff --git a/src/tee/Makefile b/src/tee/Makefile
index 7419913..f6e9135 100644
--- a/src/tee/Makefile
+++ b/src/tee/Makefile
@@ -4,7 +4,7 @@
 CFLAGS=-O2 -fno-strength-reduce
 
 ifneq (sh.exe, $(SHELL))
-DEL = rm
+DEL = rm -f
 else
 DEL = del
 endif