Remove commented out reference to old png files (riscv#2178)

Author: arichardson

src/c-st-ext.adoc

@@ -908,14 +908,11 @@ RV64
 [[rvc-instr-table0]]
 .Instruction listing for RVC, Quadrant 0
 include::images/bytefield/rvc-instr-quad0.edn[]
-//include::images/bytefield/rvc-instr-quad0.png[]
 
 [[rvc-instr-table1]]
 .Instruction listing for RVC, Quadrant 1
 include::images/bytefield/rvc-instr-quad1.edn[]
-//include::images/bytefield/rvc-instr-quad1.png[]
 
 [[rvc-instr-table2]]
 .Instruction listing for RVC, Quadrant 2
 include::images/bytefield/rvc-instr-quad2.edn[]
-//include::images/bytefield/rvc-instr-quad2.png[]