Remove BUILD_SPEC option

Thank you for this contribution to docs.nersc.gov!

If non-trivial please provide a summary of the changes here.

Submitters and reviewers please make sure

• internal links are relative and to the .md file
• avoid links named "here"
• alt-text for images is descriptive
• doesn't duplicate existing content
• spell check
• renders correctly
• does not include any identifiable information (real usernames, project names, etc)