[webcrypto] Remove support for AES 192-bit keys (1 of 2)

Removes uses of 192-bit AES keys from the LayoutTests. Since once the corresponding Chromium change lands they will no longer work.

BUG=381829

Review URL: https://codereview.chromium.org/325323004

git-svn-id: svn://svn.chromium.org/blink/trunk@176066 bbb929c8-8fbe-4397-9dbb-9b2b20218538