Unconditionally default emoji width to 2

"Emoji width" refers to the width of emoji codepoints. Since Unicode
9, they're classified as "wide" according to
TR11 (https://www.unicode.org/reports/tr11/).

Unicode 9 was released in 2016, and this slowly percolated into C
libraries and terminals. Glibc updated its default in 2.26, released
in August 2017.

Until now, we'd guess support for unicode 9 by checking the system
wcwidth function for an emoji - if it returned 2, we'd set our emoji
width to 2 as well.

However, that's a problem in the common case of using ssh to connect
to an old server - modern desktop OS, old server LTS OS, boom.

So now we instead just figure you've got a system that's *displaying*
the emoji that has been updated in the last 9 years.

In effect we're putting the burden on those who run old RHEL et al as
their client OS. They need to set $fish_emoji_width to 1.

Fixes #12500

Part of #12562

tests/test_driver.py

@@ -67,7 +67,6 @@ def makeenv(script_path: Path, home: Path) -> Dict[str, str]:
         "STY",
         "TERM",  # Erase this since we still respect TERM=dumb etc.
         "TERM_PROGRAM",
-        "TERM_PROGRAM_VERSION",
     ]:
         if var in env:
             del env[var]