refactor[cartesian]: Minor cleanup in backends

[romanc]

Description

I was reading a lot of code around DaCe/gt-codegen when debugging the new DaCe/gt4py bridge. This PR combines three cleanup commits:

• Always get stencil_ir from builder in GTBaseBackends. I've found no usage of stencil_ir being anything else than self.build.gtir if it was explicitly passed as an argument at all. There's thus no need to pass around self.build.gtir as long as we stay in the same class hierarchy.
• Avoid unnecessary indenting in generated code. Generated code is optionally formatted, but even if not, we can make sure the code doesn't look too ugly.
• Avoid double formatting of source code (if gt4py/dace is configured to do so). No need for formatting intermediate code parts because it's formatted anyway at the end.

Requirements

• All fixes and/or new features come with corresponding tests.
  Updated test accordingly.
• Important design decisions have been documented in the appropriate ADR inside the docs/development/ADRs/ folder.
  N/A

[FlorianDeconinck]

One doubt about a removed assert but I would hope we catch no gtir much earlier that this code