drdecode.a is linked in to drcachesim twice

While debugging another issue (see #6544) I realised that the `drcachesim` application has two copies of `drdecode.a`: one linked in to `drcachesim` and one linked into `libdynamorio`, which `drcachesim` also depends on.

`drdecode` is a direct dependency of `drcachesim`:
```
target_link_libraries(drcachesim drdecode drinjectlib drconfiglib drfrontendlib)
```
but if you remove it, `drdecode` still gets pulled in as a dependency of one of one of the other dependency libraries, and we still get two copies.