MDL-82476 theme: Remove non-SVG icons

Most of the PNG/JPG/GIF icons in the pix folder are redundant if an SVG
file is available, so they have been removed to simplify maintenance.

This commit removes all redundant icons from the pix/ folder. Additional
redundant icons in the icons/ folder will be addressed separately while
reviewing the FontAwesome icons and their SVG versions.

mod/assign/feedback/editpdf/tests/behat/annotate_pdf.feature

@@ -101,8 +101,8 @@ Feature: In an assignment, teacher can annotate PDF files during grading
     And I log in as "admin"
     And I am on site homepage
     And I navigate to "Plugins > Activity modules > Assignment > Feedback plugins > Annotate PDF" in site administration
-    And I upload "pix/help.png" file to "" filemanager
-    And I upload "pix/docs.png" file to "" filemanager
+    And I upload "pix/moodlelogo.png" file to "" filemanager
+    And I upload "pix/i/test.png" file to "" filemanager
     And I press "Save changes"
     And I should see "Changes saved"
     And I follow "Test ghostscript path"