Commit cd5369c added some cleanup code to `libs/ardour/session.cc`,
which drags in the type info symbol of VST3PluginInfo. When built
with `--no-vst3`, the resulting binary dies at startup:
```
ld-elf.so.1: /usr/local/lib/ardour8/libardour.so.3: Undefined symbol
"_ZTIN6ARDOUR14VST3PluginInfoE"
```
Make that code compile conditional on VST3_SUPPORT.
Occurs on FreeBSD 14.0-RELEASE, clang 16.0.6, amd64.
845171807f