Move StripRedundantLastElementsOfPath before ArgsManager class.

Summary:
PR description:

This PR attempts to contribute to "Remove gArgs" (#21005).

Main changes:

`GetDataDir()` function is moved to `ArgsManager.GetDataDirPath()`.
`GetBlocksDir()` function is moved to `ArgsManager.GetBlocksDirPath()`.

This is a backport of core#21244 [1/8]
https://github.com/bitcoin/bitcoin/pull/21244/commits/70cdf679f8e665dbdc3301873a0267fe9faa72cd

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10749