A few struct members were only ever written to and never read, which
means these were encoder only struct members. Since we only need the
ASTC decoder, these encoder only struct members were removed.

The gains in memory allocation sizes are substantial:

block_size_descriptor
Initial size: 3469056 B (~3.3 MB)
After this cl: 693168 B (~677 kB)

decimation_table
Initial size: 364896 B (~356 kB)
After this cl: 1948 B (~1.9 kB)

Change-Id: Id86c993331d2602a1106f6aaa729d1047e2ebc27
Tests: dEQP-VK.*astc*
Bug: b/150130101
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46128
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>