Theme JSON: remove unused vars in layout class (#59938)

Remove unused vars:
- $block_gap_value is immediately overwritten in the if block
- $has_block_gap_support is only used once

Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

lib/class-wp-theme-json-gutenberg.php

@@ -2728,10 +2728,8 @@ public function get_root_layout_rules( $selector, $block_metadata ) {
  $css .= '.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }';
  $css .= '.wp-site-blocks > .alignright { float: right; margin-left: 2em; }';
  $css .= '.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }';
-
- $block_gap_value = $this->theme_json['styles']['spacing']['blockGap'] ?? '0.5em';
- $has_block_gap_support = isset( $this->theme_json['settings']['spacing']['blockGap'] );
- if ( $has_block_gap_support ) {
+ // Block gap styles will be output unless explicitly set to `null`. See static::PROTECTED_PROPERTIES.
+ if ( isset( $this->theme_json['settings']['spacing']['blockGap'] ) ) {
  $block_gap_value = static::get_property_value( $this->theme_json, array( 'styles', 'spacing', 'blockGap' ) );
  $css .= ":where(.wp-site-blocks) > * { margin-block-start: $block_gap_value; margin-block-end: 0; }";
  $css .= ':where(.wp-site-blocks) > :first-child:first-child { margin-block-start: 0; }';