Simplify the SRAM layers

[RTimothyEdwards]

The SRAM DRC rule exceptions in GF180MCU are implemented in the "historical" manner by creating special layers for all the diffusion layer types inside the SRAM core cell. To cover all of the DRC rule exceptions requires 12 additional layers (covering both the 5V and 3.3V versions of the SRAM). A recent development effort in magic (version 8.3.616) created two new statements for the "drc" section of the magic tech file, "exemption" and "exception". This allows DRC rule exceptions to be implemented directly, avoiding the need for special extra layers to satisfy the SRAM DRC rule exceptions.

The fix is relatively straightforward: Remove the "sram" layers, except in the DRC section where the "sram" layers are involved in rule exceptions, use the "exception" and "exemption" statements.