Fix +build lines, remove extraneous pkg-config directive

[Crosse]

This commit should make systray build on Ubuntu 20.04 and (hopefully) Debian 11 again. I fumbled the legacy "+build" tag syntax, apparently, and forgot to remove a pkg-config directive that unconditionally required libappindicator3 to be installed.

I've tested this in the following configurations:

• Ubuntu 21.10 using Go 1.17 with and without the legacy_appindicator build tag;
• Ubuntu 20.04 using Go 1.16.2 with and without the legacy_appindicator build tag;
• Ubuntu 20.04 using Go 1.13.8 with and without the legacy_appindicator build tag.

In addition, for each of the above, I tested that the build succeeded if only one of the two libraries (appindicator3 or libayatana-appindicator3) was installed.

Note to future self: it'd probably be a good idea to enable GitHub Actions for this repo.

[Crosse]

@nange, can you try this out and see if it fixes your issue?

[nange]

@Crosse It works for me.