docs/yul: pc is dissalowed in solc yul

pc was removed from solc assembly in 0.7.0 (it introduced an unwelcome
semantic dependency on code location).

Author: d-xo

docs/yul.rst

@@ -820,8 +820,6 @@ the ``dup`` and ``swap`` instructions as well as ``jump`` instructions, labels a
 +-------------------------+-----+---+-----------------------------------------------------------------+
 | keccak256(p, n)         |     | F | keccak(mem[p...(p+n)))                                          |
 +-------------------------+-----+---+-----------------------------------------------------------------+
-| pc()                    |     | F | current position in code                                        |
-+-------------------------+-----+---+-----------------------------------------------------------------+
 | pop(x)                  | `-` | F | discard value x                                                 |
 +-------------------------+-----+---+-----------------------------------------------------------------+
 | mload(p)                |     | F | mem[p...(p+32))                                                 |