backend: removes unnecessary ValueDefinition fields

[mathetake]

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   1.541 ± 0%   1.535 ± 0%  -0.37% (p=0.015 n=6)
Compilation/zig-10      3.422 ± 0%   3.413 ± 0%  -0.27% (p=0.004 n=6)
Compilation/zz-10       14.10 ± 0%   14.08 ± 0%       ~ (p=0.310 n=6)
geomean                 4.205        4.194       -0.25%

                      │   old.txt    │              new.txt               │
                      │     B/op     │     B/op      vs base              │
Compilation/wazero-10   283.0Mi ± 0%   273.4Mi ± 0%  -3.38% (p=0.002 n=6)
Compilation/zig-10      596.7Mi ± 0%   594.7Mi ± 0%  -0.32% (p=0.002 n=6)
Compilation/zz-10       536.5Mi ± 0%   532.4Mi ± 0%  -0.76% (p=0.002 n=6)
geomean                 449.1Mi        442.4Mi       -1.50%

                      │   old.txt   │              new.txt              │
                      │  allocs/op  │  allocs/op   vs base              │
Compilation/wazero-10   485.4k ± 0%   485.2k ± 0%       ~ (p=0.394 n=6)
Compilation/zig-10      275.5k ± 0%   275.5k ± 0%       ~ (p=0.974 n=6)
Compilation/zz-10       627.0k ± 0%   627.1k ± 0%       ~ (p=0.145 n=6)
geomean                 437.7k        437.6k       -0.01%

#2182