sim/sh: Remove redundant function declaration

sim/sh: Remove redundant function declaration

Clang generates a warning if there is a function declaration/definition
with zero arguments.  Such declarations/definitions without a prototype (an
argument list) are deprecated forms of indefinite arguments
("-Wdeprecated-non-prototype").  On the default configuration, it causes a
build failure (unless "--disable-werror" is specified).

But there is another issue.  This function declaration in sim/sh/interp.c
is completely redundant.  This commit just removes that declaration.