[typescript-fetch] [BUG] Fix duplication of ModelNamePrefix in import statements

[ASterdyniak]

This fixes an issue introduced in version 7.6.0. When using the ModelNamePrefix option with typescript-fetch, the given prefix is duplicated in import statements leading to compilation failure (see issue for how to reproduce).

The root case is that the method toModelFilename that is adding prefix and suffix is called twice when generating the variables used to create import statements.

Fixes the following issue for typescript-fetch generator
#19039

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[ASterdyniak]

cc @En0s0und