As you can read in the answer to this question, each font has its own properties (for example: enabling a Regular Style
or not), as provided by the font creator.
By looking at your font (even just by looking at its name), it seems clear that it is a sub-type whose defining characteristics are precisely being (semi-)bold and italic; consequently, it does sound logical to not have the option to remove these features. If you want a non-bold, non-italic version, you should rely on the parent family (Sentinel).