[Patterns] remove irrelevant specification of pattern type schemas. (#2639)

The pattern type schemas for logical-or, null-check, constant, and
relational patterns were not meaningful because pattern type schemas
are only used in refutable contexts, and these patterns are not
allowed to appear in irrefutable contexts.

In particular, the definition of pattern type schema for constant
patterns was causing confusion because it appeared to cause a
circularity: it defined the pattern type schema for a constant pattern
to be the inferred type of the constant; however text elsewhere
specifies that the type of the constant is inferred based on the type
of the scrutinee, and the pattern type schema is used to infer the
type of the scrutinee.  However this wasn't a real problem, because
constant patterns are illegal in precisely the circumstances in whch
pattern type schemas are used.

Author: stereotype441

accepted/future-releases/0546-patterns/feature-specification.md

@@ -1570,22 +1570,16 @@ var (a, int b) = ... // Schema is `(?, int)`.
 
 The context type schema for a pattern `p` is:
 
-*   **Logical-or**: The least upper bound of the context type schemas of the
-    branches.
-
 *   **Logical-and**: The greatest lower bound of the context type schemas of the
     branches.
 
-*   **Null-check** or **null-assert**: A context type schema `E?` where `E` is
-    the context type schema of the inner pattern. *For example:*
+*   **Null-assert**: A context type schema `E?` where `E` is the context type
+    schema of the inner pattern. *For example:*
 
     ```dart
     var [[int x]!] = [[]]; // Infers List<List<int>?> for the list literal.
     ```
 
-*   **Constant**: The context type schema is the static type of the pattern's
-    value.
-
 *   **Variable**:
 
     1.  In an assignment context, the context type schema is the static type of
@@ -1604,7 +1598,7 @@ The context type schema for a pattern `p` is:
         //                                 ^- Infers List<int>.
         ```
 
-*   **Relational** or **cast**: The context type schema is `?`.
+*   **Cast**: The context type schema is `?`.
 
 *   **Parenthesized**: The context type schema of the inner subpattern.
 
@@ -1675,6 +1669,10 @@ The context type schema for a pattern `p` is:
     //                  ^-- Infer Foo<num>.
     ```
 
+The pattern type schema for logical-or, null-check, constant, and relational
+patterns is not defined, because those patterns are only allowed in refutable
+contexts, and the pattern type schema is only used in irrefutable contexts.
+
 #### Type checking and pattern required type
 
 Once the value a pattern is matched against has a static type (which means