remove-simple-social-icons-widget-css.php

<?php
add_action( 'widgets_init', 'ic_remove_simple_social_css', 999 );

function ic_remove_simple_social_css() {
	global $wp_registered_widgets;

	$widgetNames = array_keys( $wp_registered_widgets );
	$ssWidgets   = [];

	foreach ( $widgetNames as $name ) {
		if ( false !== strpos( $name, 'simple-social-icons' ) ) {
			$ssWidgets[] = $name;
		}
	}

	foreach ( $ssWidgets as $widget ) {
		$widgetObject = $wp_registered_widgets[ $widget ]['callback'][0];
		remove_action( 'wp_head', [ $widgetObject, 'css' ] );
	}
}

Removing the CSS from the simple social icons widget requires finding the widget object. This sorts through all the widgets that are registered in the WP global $wp_registered_widgets to see if the widget has been registered, and if it has, grabs the object from the registered widgets array and unregisters the action. Your mileage may vary.