Is your feature request related to a problem? Please describe.
When the editor-font-sizes array has been limited to a single item, paragraphs only have one size. If custom font sizes are also disabled, the font size picker becomes a redundant control. See below screenshot:

Describe the solution you'd like
When custom font sizes are disabled, and only a single editor-font-size is set, the font size picker should not be displayed.

For instance, given the following code:

<?php
	add_theme_support( 'editor-font-sizes', array(
		array(
			'name' => __( 'Normal', 'gutenberg-test' ),
			'shortName' => __( 'N', 'gutenberg-test' ),
			'size' => 16,
			'slug' => 'normal'
		),
	) );
	add_theme_support('disable-custom-font-sizes');

I would expect to see the following:

Describe alternatives you've considered
This could potentially be solved on a case-by-case basis with some custom JS, or by creating a custom block. However, these are not particularly good solutions compared to a simple conditional statement in the original block definition.