Cache Storage WPT: Code clean up

This change addresses a code clean up where we no longer need to create
a separate vector for storing cache names that were previously converted
from UTF8 to u16string type.

After the fix landed in the linked CL, the vector of cache names passed
to Keys() is by default of type u16string.
https://chromium-review.googlesource.com/c/chromium/src/+/6170241

Bug: 41142654
Change-Id: Ib3fa00119f3db6c4a89435c997317b057a7cc940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6417502
Reviewed-by: Evan Stade <evanstade@microsoft.com>
Reviewed-by: Rahul Singh <rahsin@microsoft.com>
Commit-Queue: Sneha Agarwal <snehagarwal@microsoft.com>
Reviewed-by: Steve Becker <stevebe@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1440571}