boards: st: stm32h7s78_dk: remove unnecessary CONFIG_SYS_HEAP_AUTO in both defconfigs

[bklaric1]

Changes

Remove the CONFIG_SYS_HEAP_AUTO=y config from both defconfig files, since it is no longer needed.

It was necessary when sram0 was smaller than 262136 bytes, as stated here.
But since #97448 increased the sram0 size to 456kB, this option is selected by default.

Tests

Tested as following:

1. Commented out CONFIG_SYS_HEAP_AUTO=y in stm32h7s78_dk_stm32h7s7xx_ext_flash_app_defconfig
2. Built and flashed a sample (dumb_http_server, since I was already testing something else with it)
3. Checked \build\zephyr\.config and confirmed that CONFIG_SYS_HEAP_AUTO=y is enabled by default

[github-actions]

Hello @bklaric1, and thank you very much for your first pull request to the Zephyr project!
Our Continuous Integration pipeline will execute a series of checks on your Pull Request commit messages and code, and you are expected to address any failures by updating the PR. Please take a look at our commit message guidelines to find out how to format your commit messages, and at our contribution workflow to understand how to update your Pull Request. If you haven't already, please make sure to review the project's Contributor Expectations and update (by amending and force-pushing the commits) your pull request if necessary.
If you are stuck or need help please join us on Discord and ask your question there. Additionally, you can escalate the review when applicable. 😊

[erwango]

Thanks.
Btw, this symbol is enabled automatically when needed, so hard setting it wasn't required anyway:

	default SYS_HEAP_SMALL_ONLY if (SRAM_SIZE <= 256)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud