resourceloader: Remove checkCssHandles for modules without styles

resourceloader: Remove checkCssHandles for modules without styles

When execute() begins, we are in a stack that is suitable for
script execution. In the case of styles needing to be inserted,
we end that stack by scheduling the styles insertion via
requestAnimationFrame. When that styles insertion is completed,
we are currently continuing the script execution in that same
stack, which is bad for performance and also creates a confusing
call stack, given that none of that code is paint or animation
related.

Before we can address that, e.g. by unwinding the stack via
requestIdleCallback, we first need to clean up the code to not
needlessly involve checkCssHandles for modules without styles.

Most modules are small and have only scripts, no styles, so this
is important to keep in a single stack and rapidly executed in
one batch - without interuption.

This commit moves the logic that was previously inside checkCssHandles
to the bottom of execute(), which is where checkCssHandles used to
be called from.

Bug: T202703
Change-Id: I13a996e01b48bab477e68ce933a5e2ef05b361aa