Remove unnecessary using statements from vectorized functions

[rayleigh]

Summary:

Some of the functions vectorized in #202 have an unnecessary using stan::math:: statement that can be removed.

Description:

See above.

Reproducible Steps:

Look in the code. If it's including a function defined in stan/math/prim/scal/fun/ and has an using stan::math:: statement, remove the using statement.

Current Output:

The using statement is there.

Expected Output:

The using statement should be removed.

Additional Information:

Provide any additional information here.

Current Version:

v2.12.0

[bob-carpenter]

This is a much bigger issue. When you're in a namespace, you don't need the namespace qualification. I'll go through and get rid of all the using statements of the form using stan::math::X; and all the qualifications stan::math::.