Remove "Segoe UI Symbol" from font stack

[simurai]

This removes Segoe UI Symbol from the $body-font stack.

Reasoning

See issue https://github.com/github/design-systems/issues/710 and more specifically this comment https://github.com/github/design-systems/issues/710#issuecomment-533818820.

In short Segoe UI Symbol is used as a fallback for native emoji support. Removing Segoe UI Symbol allows users to have other fonts that use the same "private" unicode characters. For example to display certain chinese symbols.

Concerns

I think it's only an issue for people still on Windows 7 and only affects non-markdown areas on dotcom.. like the title of an issue.

/cc @muan @primer/ds-core

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[simurai]

I tagged this as patch release + Tag: Bug Fix although at the same time it could be a "breaking change" and therefore part of a major release? As I understood it, it's pretty important to get this released as soon as possible. So maybe just a patch release is fine?

[simurai]

Ok, I'm gonna merge this already into #905 and we can keep it as the only change. So that we don't have to test anything else.