MDEV-25916: Compilation failed for compile-pentium64-gcov script

- Commit e3bffd5 introduced the change
- Fixed with commit 2d85714 in 10.4 (no removal of
`Wimplicit-fallthrough=2`)
- Fixed with commit 4a75b48 in 10.5+

- Closing PR #2817
Reviewed by: <monty@mariadb.org>

BUILD/SETUP.sh

@@ -141,7 +141,7 @@ elif [ "x$warning_mode" = "xmaintainer" ]; then
   debug_extra_cflags="-g3"
 else
 # Both C and C++ warnings
-  warnings="-Wall -Wextra -Wunused -Wwrite-strings -Wno-uninitialized -Wno-strict-aliasing -Wimplicit-fallthrough=2 -Wformat-security -Wvla"
+  warnings="-Wall -Wextra -Wunused -Wwrite-strings -Wno-uninitialized -Wno-strict-aliasing -Wformat-security -Wvla"
 
 # For more warnings, uncomment the following line
 # warnings="$warnings -Wshadow"