Remove the ignored second argument from Config::get() calls

Problem/Motivation

Several call sites pass a default value as a second argument to Config::get():

$default_case = $config->get('default_case', 'standard');
$enable_attributes = $config->get('enable_attributes', FALSE);

Drupal\Core\Config\ConfigBase::get() is declared as get($key = '') and takes a single argument, so the second one is silently discarded and the intended default never applies. The code reads as if a fallback were in place when there is none.

Affected call sites on 4.0.x-dev:

• src/Form/BlockClassBulkOperationsForm.php:42 — get('enable_attributes', FALSE)
• src/Form/BlockClassBulkOperationsForm.php:71 — get('enable_id_replacement', FALSE)
• src/Service/BlockClassHelperService.php:80 — get('default_case', 'standard')
• src/Service/BlockClassHelperService.php:813 — get('default_case', 'standard')
• src/Service/BlockClassHelperService.php:1129 — get('default_case', 'standard')
• block_class.module:133 — get('default_case', 'standard')

Steps to reproduce

Not user-facing — block_class.settings ships default_case, enable_attributes and enable_id_replacement in config/install, so the discarded defaults are never needed in practice. This is a readability and correctness clean-up, not a behaviour fix.

Proposed resolution

Drop the second argument at all six call sites, as proposed in MR !5 on the closed issue #3279758. Where a fallback is genuinely wanted, make it explicit at the call site instead.

Remaining tasks

• Open a merge request against 4.0.x, crediting hswong3i for the original change in MR !5.
• Review and test.

User interface changes

None.

API changes

None.

Data model changes

None.