apps: Remove use of blue filter

There is a blue colored filter on icons in the apps page. It has been agreed that this does not look pleasing nor does it adds value. Remove it.

• Remove the CSS styling in plinth.css related to blue color filter on the app page icons.
• Remove the CSS styling causing the transition effects too.
• Update screenshot in README.md

This issue is nominated for Contributor Invite:

• Task Area: Coding
• Effort Rating: 1/5
• Skills Required: CSS only

Original description

The blue filter for enabled apps seems a bit strange to me.

• The Home page doesn't have this color filter. Was there a reason for this page to be different?
• Also I think it removes too much color from the icons. Maybe a more gradual filter would be ok.

This is just my subjective opinion of course.