Drop references to deleted chapter (riscv#2176)

The chapter was removed in 01e31a0.

Author: arichardson

src/intro.adoc

@@ -315,8 +315,7 @@ extensions. Non-standard extensions are either custom extensions, that
 use only custom encodings, or _non-conforming_ extensions, that use any
 standard or reserved encoding. Instruction-set extensions are generally
 shared but may provide slightly different functionality depending on the
-base ISA. <<extending>> describes various ways
-of extending the RISC-V ISA. We have also developed a naming convention
+base ISA. We have also developed a naming convention
 for RISC-V base instructions and instruction-set extensions, described
 in detail in <<naming>>.
 
@@ -495,7 +494,7 @@ Perhaps more importantly, by condensing our base ISA into a subset of
 the 32-bit instruction word, we leave more space available for
 non-standard and custom extensions. In particular, the base RV32I ISA
 uses less than 1/8 of the encoding space in the 32-bit instruction word.
-As described in <<extending>>, an implementation that does not require support
+An implementation that does not require support
 for the standard compressed instruction extension can map 3 additional non-conforming
 30-bit instruction spaces into the 32-bit fixed-width format, while preserving
 support for standard &#8805;32-bit instruction-set

src/rv-32-64g.adoc

@@ -41,9 +41,7 @@ added instructions tend to be more domain-specific and only provide
 benefits to a restricted class of applications, e.g., for multimedia or
 security. Unlike most commercial ISAs, the RISC-V ISA design clearly
 separates the base ISA and broadly applicable standard extensions from
-these more specialized additions. <<extending>>
-has a more extensive discussion of ways to add extensions to the RISC-V
-ISA.
+these more specialized additions.
 
 <<<