Remove unnecessary log alias for logrus inport

Some time ago [we had a discussion](!934 (comment 91891281))
about why we're aliasing some of the imports of `github.com/sirupsen/logrus`
package with a `log` alias.

At that moment we've been doing that in 17 from 68 cases when logrus
was imported in Runners codebase.

The conclusion was this aliasing is unnecessary, because:
- we relay on logrus interface so much that moving to another logging
  library will be not so simple as switching the import name;
- we're totally not consistent in the alias usage which makes the code
  even more messy.

Three GitLab Runner maintainers agreeded hat there is no reason to use
the aliasing and that we should just use `logrus` everywhere.

But no action was don since then. The cleanup was left for a future.

Unfortunately, because GitLab Runner is one of our biggest projects in
Go, for some people it becomes a source of suggestions of how certain
things should be handled. For example here is a discussion in a new
project that creates a go-based tool for our Serverless integration:

gitlabktl!9 (comment 152472275)

The most important statement is:

> perhaps we could leave it as is since it is how we are exposing
  logrus in the runner project

Because we already know that it should be changed and others start to
look on Runner's code to find good patterns, let's finally cleanup
the `log` aliasing mess.
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