DSLX: Remove proc initial value specification from the BUILD flow

To convert a DSLX proc into IR, we need to specify an initial value for the proc's state. That specification is currently done in the BUILD file, which is fragile. Consider the following example:
```
xls_dslx_opt_ir(
name = "aes_128_ctr",
dslx_top = "aes_128_ctr",
ir_conv_args = {
"top_proc_initial_state": "'(0, (0, [0, 0, 0, 0], 0, 0), 0, 0)'",
},
library = ":aes_128_ctr_dslx",
)
```

First of all, this is simply hard to read or parse for human eyes, and secondly, it's hard to maintain: what if the state changes for aes_128_ctr? We need to figure out some way to remove initial values from this flow.