devel/janet-lsp: set NO_ARCH and remove change to syspath in janet-lsp.in

[dave_freedave.net]

Summary

These are non-essential changes, but they clean things up:

• set NO_ARCH to only build package once for all architectures
• don't repeat file for ${INSTALL_*} targets
• remove build dependency of janet (now that jpm correctly requires anyway)
• remove the (put root-env :syspath ...) as the path of interpreter auto picks that path up anyway

Test Plan

As this doesn't actually effect the real artifact janet-lsp.jimage there isn't much need but I did the following anyway:

• make test
• installed in stable/15 and made sure it still works in neovim

[dave_freedave.net]

output from poudriere testport ... devel/janet-lsp: {F141925749}