fix: remove global SPIClass typedef from HardwareSPI API

[Aymane-ST]

Summary

Remove the global SPIClass typedef alias from HardwareSPI.h.

Why

HardwareSPI is already the SPI type exposed by ArduinoCore-API.

Some cores, such as STM32, already define their own SPIClass type or naming conventions at core level. Providing a global SPIClass alias in the shared API can therefore:

• create redefinition or naming conflicts,
• blur the boundary between the generic API type and core-specific SPI implementations,
• force a naming choice that should remain under core control.

Removing the alias keeps the generic API explicit and lets each core decide how its concrete SPI class should be named.

Compatibility

This change may affect code that depends on SPIClass being provided directly by ArduinoCore-API.

Cores and libraries using HardwareSPI directly are unaffected.