Do not show UI for changing knit directory for R Markdown website files

[jmcphers]

System details

RStudio Edition : Desktop
RStudio Version : 1.1.359
OS Version      : macOS 10.13.3
R Version       : 3.4.2

Steps to reproduce the problem

1. Create a new R Markdown website project
2. Open index.Rmd
3. Click the drop menu next to the Knit button in the editor toolbar

Describe the problem in detail

A Knit in Directory menu exists, and you can choose different knit directories. However, none of the options have any effect. This is because you can't change the knit directory when rendering websites. See #2158.

Describe the behavior you expected

This menu shouldn't appear in R Markdown files associated with websites.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs, per https://github.com/rstudio/rstudio/wiki/Issue-Grooming. Thank you for your contributions.

[stale]

This issue has been automatically closed due to inactivity.