Refactor: Replace custom environmentVariable with native qEnvironmentVariable

[rajuan34]

Description

This PR resolves an old TODO in the codebase regarding environment variable fetching.

Previously, the project used a custom environmentVariable function as a fallback to qgetenv. There was a comment in src/shared/FileSystem.h stating:
// TODO: Use qEnvironmentVariable function when we are able to use Qt 5.10 version.

Since the project now uses a modern Qt version that fully supports qEnvironmentVariable(), this custom implementation is no longer necessary. This PR removes the legacy tech debt and switches to the native Qt function.

Changes Made

• Removed the custom environmentVariable function definition and declaration from src/shared/FileSystem.cpp and src/shared/FileSystem.h.
• Updated UserFolders() in src/shared/FileSystem.h to use qEnvironmentVariable().
• Updated TopBar::DataMigration() in src/frontend/TopBar.cpp to use qEnvironmentVariable().
• Removed the associated TODO comment.

Testing

This is a safe refactor. qEnvironmentVariable() is the standard, cross-platform Qt method for fetching environment variables and behaves identically to the custom qgetenv wrapper it replaces.

[codayon]

I suggest checking the develop or new-design branches first to see whether this patch is still needed.

[mominul]

@rajuan34 Hi, thanks for your contribution! But unfortunately, this has been already implemented in the develop branch