Remove the importlib-metadata dependency

This should complete a previous attempt to remove dependency on the
package, unblocking use of `awslambdaric` in projects that use a newer
version of `importlib-metadata`. Version 2 of `importlib-metadata` is
reasonably old - version 4.4 is present in Python 3.10 for instance.

Motivation for the change
=========================
I think issue #61 covers it well, but essentially it becomes difficult
to use `awslambdaric` in a virtualenv with other dependencies that
expect a newer version of `importlib-metadata`:

#61

`awslambdaric` isn't supposed to need `importlib-metadata` anyway
=================================================================
Previous work (in August of 2021, with version 1.2.2) removed all
dependency on the `importlib-metadata` package, but accidentally left
`importlib-metadata` as a full dependency of this package:

- #55

Most recent version still has `importlib-metadata` dependency
=============================================================
```
$ pip show awslambdaric
Name: awslambdaric
Version: 2.0.4
...
Requires: importlib-metadata, simplejson
```

Author: DavidCain

requirements/base.txt

@@ -1,2 +1 @@
 simplejson==3.17.2
-importlib-metadata ~= 2.0 ; python_version < "3.8"