Remove references to specific runtimes in frontend

[duongnt]

Issue
There are hardcoded references to specific runtimes (airflow, kfp) in the frontend code. Couple of issues with this:

• bringing your own runtimes would require changing frontend code (e.g. adding a new icon for a new runtime)
• frontend code is more bloated than necessary

Grepping for kfp or airflow returns those hits:

packages/pipeline-editor/src/PipelineEditorWidget.tsx:          pipelineRuntimeName === 'kfp'
packages/pipeline-editor/src/PipelineService.tsx:export const KFP_SCHEMA = 'kfp';
packages/pipeline-editor/src/index.ts:                runtime.name === 'kfp' ? 2 : runtime.name === 'airflow' ? 3 : 4
packages/pipeline-editor/src/index.ts:            runtime.name === 'kfp' ? 31 : runtime.name === 'airflow' ? 32 : 33
packages/pipeline-editor/src/pipeline-hooks.ts:      case 'kfp':
packages/services/src/test/application.spec.ts:          'kfp',
packages/ui-components/src/icons.tsx:  name: 'elyra:kfp',
packages/pipeline-editor/src/PipelineEditorWidget.tsx:  airflowIcon
packages/pipeline-editor/src/PipelineEditorWidget.tsx:            : pipelineRuntimeName === 'airflow'
packages/pipeline-editor/src/PipelineEditorWidget.tsx:            ? airflowIcon
packages/pipeline-editor/src/PipelineService.tsx:            {response['platform'] == 'airflow' ? (
packages/pipeline-editor/src/RuntimesWidget.tsx:    if (metadata.schema_name === 'airflow' && metadata_props) {
packages/pipeline-editor/src/index.ts:                runtime.name === 'kfp' ? 2 : runtime.name === 'airflow' ? 3 : 4
packages/pipeline-editor/src/index.ts:            runtime.name === 'kfp' ? 31 : runtime.name === 'airflow' ? 32 : 33
packages/pipeline-editor/src/pipeline-hooks.ts:  airflowIcon,
packages/pipeline-editor/src/pipeline-hooks.ts:      case 'airflow':
packages/pipeline-editor/src/pipeline-hooks.ts:        category.image = IconUtil.encode(airflowIcon);
packages/pipeline-editor/src/pipeline-hooks.ts:  const runtimeIcons = [kubeflowIcon, airflowIcon];
packages/services/src/test/application.spec.ts:          'airflow'
packages/ui-components/src/icons.tsx:import airflowSvg from '../style/icons/airflow.svg';
packages/ui-components/src/icons.tsx:export const airflowIcon = new LabIcon({
packages/ui-components/src/icons.tsx:  name: 'elyra:airflow',
packages/ui-components/src/icons.tsx:  svgstr: airflowSvg

Describe the solution you'd like
Remove those hardcoded references. Frontend should be agnostic to the list of available runtimes.

Describe alternatives you've considered
Adding even more hardcode refs?

[akchinSTC]

addressed by : #2135