Regression: The --no-compile flag to dbt docs generate is no longer supported

[drewbanin]

Describe the bug

I ran dbt docs generate --no-compile and received an error that the manifest was not present:

2020-02-04 02:04:58,798143 (MainThread): Traceback (most recent call last):
  File "/Users/drew/fishtown/dbt/core/dbt/main.py", line 81, in main
    results, succeeded = handle_and_check(args)
  File "/Users/drew/fishtown/dbt/core/dbt/main.py", line 159, in handle_and_check
    task, res = run_from_args(parsed)
  File "/Users/drew/fishtown/dbt/core/dbt/main.py", line 212, in run_from_args
    results = task.run()
  File "/Users/drew/fishtown/dbt/core/dbt/task/generate.py", line 208, in run
    'self.manifest was None in run!'
dbt.exceptions.InternalException: self.manifest was None in run!

I observed this behavior when running dbt on Snowflake, but unsure if that's pertinent.

Expected behavior

The --no-compile flag should prevent dbt from compiling the project while still producing a catalog. If this is no longer possible, we should remove the flag.

The output of dbt --version:
I observed this with dbt v0.15.2 and dev/barbara-gittings