chore: do not clean docs during build

[trivikr]

Issue #, if available:
Refs: aws/aws-sdk-js-v3#3138 (comment)

Description of changes:
The build:docs command now calls typedoc directly, so that it can be used in internal release automation.

Reasons for removing clean:docs call:

• The build:docs command should do work only what it's asked to do. Cleaning docs is an additional work.
• The docs may not have been generated in the default docs folder. We'll decide later if clean:docs script should be removed altogether.
• The typedoc dependency is being moved to root package.json in chore(scripts): remove devDeps defined in monorepo root aws/aws-sdk-js-v3#3138, so typedoc will not be available in node_modules for each client and they need to use build:docs command.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.