Revert "syntax: simplify unset-or-null with null default to just unset"

Per #970, even if the change is technically correct, it can easily lead
to confusion for users or unintentional bugs when they tweak the code
without noticing the subtlety between null (empty) and unset values.

This reverts commit e8ee47f.

Fixes #970.

syntax/simplify.go

@@ -16,7 +16,6 @@ import "bytes"
 //	Remove redundant quotes                  [[ "$var" == str ]]
 //	Merge negations with unary operators     [[ ! -n $var ]]
 //	Use single quotes to shorten literals    "\$foo"
-//	Remove redundant param expansion colons  ${foo:-}
 func Simplify(n Node) bool {
 	s := simplifier{}
 	Walk(n, s.visit)
@@ -38,10 +37,6 @@ func (s *simplifier) visit(node Node) bool {
 		x.Index = s.removeParensArithm(x.Index)
 		// don't inline params - same as above.
 
-		if x.Exp != nil && x.Exp.Op == DefaultUnsetOrNull && x.Exp.Word == nil {
-			s.modified = true
-			x.Exp.Op = DefaultUnset
-		}
 		if x.Slice == nil {
 			break
 		}

syntax/simplify_test.go

@@ -67,10 +67,6 @@ var simplifyTests = [...]simplifyTest{
 	{"\"fo\\`o\"", "'fo`o'"},
 	noSimple(`fo"o"bar`),
 	noSimple(`foo""bar`),
-
-	// param expansions
-	{`${foo:-}`, `${foo-}`},
-	noSimple(`${foo:-bar}`),
 }
 
 func TestSimplify(t *testing.T) {