Remove schema override for variable default value (#1536)

## Changes

This PR:
1. Removes the custom added in
https://github.com/databricks/cli/pull/1396/files for
`variables.*.default`. It's no longer needed because with complex
variables (https://github.com/databricks/cli/pull/1467) `default` has a
type of any.
2. Retains, and extends the override on `targets.*.variables.*`. Target
override values can now be complex objects, not just primitive values.

## Tests
Manually

Before:
Only primitive types were allowed.

<img width="436" alt="Screenshot 2024-06-27 at 3 58 34 PM"
src="https://github.com/databricks/cli/assets/88374338/de55be5c-9236-4ccc-b529-97ce7201b8e8">

After:
An empty JSON schema is generated. All YAML values are acceptable.

<img width="453" alt="Screenshot 2024-06-27 at 3 57 15 PM"
src="https://github.com/databricks/cli/assets/88374338/7534b770-563f-4efc-b9c6-0af526dcc705">

cmd/bundle/schema.go

@@ -11,54 +11,6 @@ import (
 	"github.com/spf13/cobra"
 )
 
-func overrideVariables(s *jsonschema.Schema) error {
-	// Override schema for default values to allow for multiple primitive types.
-	// These are normalized to strings when converted to the typed representation.
-	err := s.SetByPath("variables.*.default", jsonschema.Schema{
-		AnyOf: []*jsonschema.Schema{
-			{
-				Type: jsonschema.StringType,
-			},
-			{
-				Type: jsonschema.BooleanType,
-			},
-			{
-				Type: jsonschema.NumberType,
-			},
-			{
-				Type: jsonschema.IntegerType,
-			},
-		},
-	})
-	if err != nil {
-		return err
-	}
-
-	// Override schema for variables in targets to allow just specifying the value
-	// along side overriding the variable definition if needed.
-	ns, err := s.GetByPath("variables.*")
-	if err != nil {
-		return err
-	}
-	return s.SetByPath("targets.*.variables.*", jsonschema.Schema{
-		AnyOf: []*jsonschema.Schema{
-			{
-				Type: jsonschema.StringType,
-			},
-			{
-				Type: jsonschema.BooleanType,
-			},
-			{
-				Type: jsonschema.NumberType,
-			},
-			{
-				Type: jsonschema.IntegerType,
-			},
-			&ns,
-		},
-	})
-}
-
 func newSchemaCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "schema",
@@ -79,9 +31,9 @@ func newSchemaCommand() *cobra.Command {
 			return err
 		}
 
-		// Override schema for variables to take into account normalization of default
-		// variable values and variable overrides in a target.
-		err = overrideVariables(schema)
+		// Target variable value overrides can be primitives, maps or sequences.
+		// Set an empty schema for them.
+		err = schema.SetByPath("targets.*.variables.*", jsonschema.Schema{})
 		if err != nil {
 			return err
 		}