fix(form-field): Remove experimental gap mixin (#1601)

Best to leave layout decisions to developers.

Author: trimox

packages/material-components-web/extend/form-field/_mixins.scss

@@ -0,0 +1,5 @@
+.ngx-mdc-form-field--fluid {
+  display: flex;
+  flex: auto;
+  flex-direction: column;
+}

packages/material-components-web/extend/form-field/mdc-form-field.scss

@@ -1,5 +1,5 @@
 @import "material-components-web";
-@import "extend/form-field/mdc-form-field";
+@import "extend/mdc-form-field";
 @import "extend/mdc-button";
 @import "extend/mdc-chips";
 @import "extend/mdc-fab";

packages/material-components-web/extend/mdc-form-field.scss

@@ -365,10 +365,6 @@ export class MdcSelect implements OnInit, ControlValueAccessor, OnDestroy {
 
     // Force setter to be called in case id was not specified.
     this.id = this.id;
-
-    if (this._parentFormField) {
-      _parentFormField.elementRef.nativeElement.classList.add('ngx-mdc-form-field');
-    }
   }
 
   ngOnInit(): void {

packages/material-components-web/material.scss

@@ -353,10 +353,6 @@ export class MdcTextField extends _MdcTextFieldMixinBase implements AfterViewIni
 
     // Force setter to be called in case id was not specified.
     this.id = this.id;
-
-    if (this._parentFormField) {
-      _parentFormField.elementRef.nativeElement.classList.add('ngx-mdc-form-field');
-    }
   }
 
   ngAfterViewInit(): void {