gdb: fix shellcheck warnings SC2034 (unused variable) in gdbarch.sh

shellcheck reports:

    In gdbarch.sh line 139:
                    fallbackdefault="0"
                    ^-------------^ SC2034: fallbackdefault appears unused. Verify use (or export if used externally).

Indeed, the `fallbackdefault` variable appears to be unused, remove the
code that sets it.

gdb/ChangeLog:

	* gdbarch.sh: Remove code that sets fallbackdefault.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-04-29  Simon Marchi  <simon.marchi@efficios.com>
+
+	* gdbarch.sh: Remove code that sets fallbackdefault.
+
 2020-04-29  Simon Marchi  <simon.marchi@efficios.com>
 
 	* gdbarch.sh: Use shell operators && and || instead of

gdb/gdbarch.sh

@@ -122,23 +122,6 @@ EOF
 		esac
 	    esac
 
-	    # PREDEFAULT is a valid fallback definition of MEMBER when
-	    # multi-arch is not enabled.  This ensures that the
-	    # default value, when multi-arch is the same as the
-	    # default value when not multi-arch.  POSTDEFAULT is
-	    # always a valid definition of MEMBER as this again
-	    # ensures consistency.
-
-	    if [ -n "${postdefault}" ]
-	    then
-		fallbackdefault="${postdefault}"
-	    elif [ -n "${predefault}" ]
-	    then
-		fallbackdefault="${predefault}"
-	    else
-		fallbackdefault="0"
-	    fi
-
 	    #NOT YET: See gdbarch.log for basic verification of
 	    # database
 
@@ -156,8 +139,8 @@ EOF
 
 fallback_default_p ()
 {
-    ( [ -n "${postdefault}" ] && [ "x${invalid_p}" != "x0" ] ) \
+    { [ -n "${postdefault}" ] && [ "x${invalid_p}" != "x0" ]; } \
-	|| ( [ -n "${predefault}" ] && [ "x${invalid_p}" = "x0" ] )
+	|| { [ -n "${predefault}" ] && [ "x${invalid_p}" = "x0" ]; }
 }
 
 class_is_variable_p ()