emsdk_env.fish works but shows errors

Using `emsdk_env.fish` has been showing error in console for a while. I never looked before what was it because there is a lot of noise in the output due to long `PATH` variable and because the environment is still set up correctly - but it seems to be due to usage of `unset` command which doesn't exist in fish:

```shell
> source ~/emsdk/emsdk_env.fish
Setting up EMSDK environment (suppress these messages with EMSDK_QUIET=1)
Adding directories to PATH:
PATH += /home/rreverser/emsdk
PATH += /home/rreverser/emsdk/upstream/emscripten

Setting environment variables:
PATH = <...>
EMSDK = /home/rreverser/emsdk
EMSDK_NODE = /home/rreverser/emsdk/node/16.20.0_64bit/bin/node
Clearing existing environment variable: EMSDK_PATH
unset: command not found
~/emsdk/emsdk_env.fish (line 1):
export PATH="<...>"; unset EMSDK_PATH;
^~~~^
from sourcing file ~/emsdk/emsdk_env.fish
```