chore: remove unecessary check if node exists

[vitormattos]

Pull Request Description

The method get already do this and throw an exception when the file don't exists.

Was possible remove all nodeExists.

I also made small refactors after remove all nodeExists.

Related Issue

• Reduce IO when create folder #5321

Close #5321

Pull Request Type

• Refactoring (no functional changes, no api changes)

Pull request checklist

• Did you explain or provide a way of how can we test your code ?
• If your pull request is related to frontend modifications provide a print of before and after screen
• Did you provide a general summary of your changes ?
• Try to limit your pull request to one type, submit multiple pull requests if needed
• I implemented tests that cover my contribution

[vitormattos]

The broke tests aren't about LibreSign, already was fixed at guests app.

{
	"app": "PHP",
	"message": "Declaration of OCA\\Guests\\AppConfigOverwrite::getValue(string $app, string $key, ?string $default = null): string must be compatible with OC\\AppConfig::getValue($app, $key, $default = '') at \/var\/www\/html\/apps-extra\/guests\/lib\/AppConfigOverwrite.php#22"
}

[vitormattos]

The broke tests aren't about LibreSign, already was fixed at guests app.

{
	"app": "PHP",
	"message": "Declaration of OCA\\Guests\\AppConfigOverwrite::getValue(string $app, string $key, ?string $default = null): string must be compatible with OC\\AppConfig::getValue($app, $key, $default = '') at \/var\/www\/html\/apps-extra\/guests\/lib\/AppConfigOverwrite.php#22"
}

Fixed, was created a new release with the fix at app Guests and I made a rebase at this branch to be possible restart the CI.

After your last review I only made this commit:

7a4f366

[vitormattos]

/backport to stable31

[vitormattos]

/backport to stable30