Remove fixed-time division (ISA-110) (#1670)

After further investigation, the feature is not needed for security application.

For safety applications, variable-time division is only one of several sources of unpredictability and it does not make no real sense to fix it.

docs/02_cva6_requirements/cva6_requirements_specification.rst

@@ -303,16 +303,6 @@ independent requirements.
 |                                   | **Zifencei** extension, version   |
 |                                   | 2.0.                              |
 +-----------------------------------+-----------------------------------+
-| ISA‑110                           | | As an **option**, the duration  |
-|                                   |   of instructions shall be        |
-|                                   |   independent from the operand    |
-|                                   |   values.                         |
-|                                   | | *Unlike other options, this one |
-|                                   |   can be design-time (selected    |
-|                                   |   before compiling the RTL) or    |
-|                                   |   run-time (selected through a    |
-|                                   |   register).*                     |
-+-----------------------------------+-----------------------------------+
 | ISA-120                           | CVA6 should support as an         |
 |                                   | **option** the **Zba**, **Zbb**,  |
 |                                   | **Zbc** and **Zbs** extensions    |
@@ -332,9 +322,6 @@ independent requirements.
 Note to ISA-60 and ISA-70: CV64A6 cannot support the D extension with
 the F extension.
 
-Note to ISA-110: In the current design, the duration of the division
-is data-dependent, which can be a security issue.
-
 .. _privileges_and_virtual_memory:
 
 Privileges and virtual memory