Remove work-around for pcrel issue

With rust-lang/rust#62281, rust compiles in mcmodel=medium mode
by default and this work-around is no longer needed.

memory-k210.x

@@ -2,10 +2,8 @@ _max_hart_id = 1;
 
 MEMORY
 {
-    /* Workaround for the pcrel issue, as described here:
-       https://github.com/rust-embedded/riscv-rt/issues/25#issuecomment-491518168 */
-    SRAM : ORIGIN = 0xffffffff80000000, LENGTH = 6M
-    AI_SRAM : ORIGIN = 0xffffffff80600000, LENGTH = 2M
+    SRAM : ORIGIN = 0x80000000, LENGTH = 6M
+    AI_SRAM : ORIGIN = 0x80600000, LENGTH = 2M
     SRAM_NOCACHE : ORIGIN = 0x40000000, LENGTH = 6M
     AI_SRAM_NOCACHE : ORIGIN = 0x40600000, LENGTH = 2M
 }