fast_import: Fix shared_buffers setting (#10837)

In commit 9537829ccd I made shared_buffers be derived from the system's
available RAM. However, I failed to remove the old hard-coded
shared_buffers=10GB settings, shared_buffers was set twice. Oopsie.

compute_tools/src/bin/fast_import.rs

@@ -211,7 +211,6 @@ impl PostgresProcess {
             .args(["-p", &format!("{port}")])
             .args(["-c", "wal_level=minimal"])
             .args(["-c", &format!("shared_buffers={shared_buffers_mb}MB")])
-            .args(["-c", "shared_buffers=10GB"])
             .args(["-c", "max_wal_senders=0"])
             .args(["-c", "fsync=off"])
             .args(["-c", "full_page_writes=off"])