[web] Remove safari-pinned-tab.svg

Summary:
Addresses https://linear.app/comm/issue/ENG-963/remove-old-squad-cal-favicon-on-safari

It used to be that Safari didn't display favicons for each tab and instead showed the page title. However, if a tab was pinned--or there wasn't enough space to show an abbreviated title--it would display the first letter of the page title:

Apple gave the option of providing a black square SVG (safari-pinned-tab.svg) to be displayed for "pinned" tabs and the ability to specify a tint color for "active" tabs.

However, recent versions of Safari display favicons for tabs across the board (it's no longer even possible to disable them), and including safari-pinned-tab.svg is no longer necessary... but Safari will still use it if it's provided. Based on what I've read, it seems like the only reason to specify safari-pinned-tab.svg is if you want something other than the default favicon to be displayed on just Safari. I don't think that's something we care about.

Test Plan: Ensured correct favicon displayed in Safari/Chrome after clearing cache.

Reviewers: yayabosh, palys-swm, def-au1t

Reviewed By: yayabosh, palys-swm

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4358