[R] Revisit binding_format_datetime and remove manual casting

[asfimport]

This is a follow-up issue to revisit the casting step in format once #12240 gets merged.

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg

Related issues:

• [R] fix behaviour when converting timestamps with "" as tzone (depends upon)

_{Note: This issue was originally created as ARROW-15762. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[asfimport]

Apache Arrow JIRA Bot:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per project policy. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.