refactor(dashboard): remove inline widget flags from create command

Remove all 8 --widget-* flags from 'dashboard create', leaving it as
a clean 'sentry dashboard create [<org/project>] <title>' with 0 flags
for core functionality.

The inline widget creation is deferred to the planned 'dashboard widget
add' command in the follow-up PR. The widget type infrastructure
(parseAggregate, parseWidgetInput, assignDefaultLayout, etc.) remains
in src/types/dashboard.ts as pre-staged code for that PR.

Removed:
- buildInlineWidget() function and all widget validation logic
- 8 flag definitions (widget-title, widget-display, widget-dataset,
  widget-query, widget-where, widget-group-by, widget-sort, widget-limit)
- 3 widget-related tests
- ~190 lines net reduction

Author: BYK