Fix so rTerm is undefined when deleting terminal

[andycraig]

Fixes #402

What problem did you solve?

If session watcher is on but R: Always Use Active Terminal is off, closing an R terminal created by vscode-R does not undefine rTerm. This means that future commands that should create a new R terminal instead do nothing.

How can I check this pull request?

1. Enable setting R: Session Watcher
2. Disable setting R: Always Use Active Terminal
3. Create file temp.R with content x <- 1
4. Command R: Run Selection/Line
5. Observe R Interactive terminal is created and line is sent to terminal
6. Close R Interactive by clicking on the terminal trashcan icon. Terminal should now be 1: cmd (if on Windows)
7. Command R: Run Selection/Line

With PR

Terminal R Interactive is created and line is sent to terminal.

Before PR

Nothing happens.

Notes

I came up with this after putting a breakpoint on the original rTerm = undefined line and discovering that it was never reached.

[renkun-ken]

LGTM