Remove --dump-ast from help text and README

This flag was previously removed.

Change-Id: I8b6ed8dd8966fcf7b587a65e78dcc06c2b2a9cd2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68240
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/samples/main.cc b/samples/main.cc
index ca2c456..29835f4 100644
--- a/samples/main.cc
+++ b/samples/main.cc
@@ -108,7 +108,6 @@
                                 renamer
                                 robustness
   --parse-only              -- Stop after parsing the input
-  --dump-ast                -- Dump the generated AST to stdout
   --disable-workgroup-init  -- Disable workgroup memory zero initialization.
   --demangle                -- Preserve original source names. Demangle them.
                                Affects AST dumping, and text-based output languages.
diff --git a/src/reader/spirv/README.md b/src/reader/spirv/README.md
index e5f2f0d..f782e21 100644
--- a/src/reader/spirv/README.md
+++ b/src/reader/spirv/README.md
@@ -7,9 +7,6 @@
 
 It's usable from the Tint command line:
 
-    # Dump the Tint AST after reading SPIR-V.
-    tint --dump-ast --parse-only a.spv
-
     # Translate SPIR-V into WGSL.
     tint --format wgsl a.spv