NUCLEO_L476RG: GCC_ARM ld file fix

Following
Merge pull request #4063 from LMESTM/17q2_L4_bootloader
the NUCLEO_L476RG binairies could not boot anymore.

The change done in #4063 was derived from work on NUCLEO_L429ZI target
which supports uvisor. The VECTORS defintiion is introduced as part of
uvisor support and requires further changes in ld file which were missing.
As uvisor is not considered yet, we remove VECTORS for now and will
introduce only when needed.

Author: LMESTM

targets/TARGET_STM/TARGET_STM32L4/TARGET_STM32L476xG/device/TOOLCHAIN_GCC_ARM/STM32L476XX.ld

@@ -9,8 +9,7 @@
 /* Linker script to configure memory regions. */
 MEMORY
 { 
-  VECTORS (rx) : ORIGIN = MBED_APP_START, LENGTH = 0x400
-  FLASH (rx)   : ORIGIN = MBED_APP_START + 0x400, LENGTH = MBED_APP_SIZE - 0x400
+  FLASH (rx)   : ORIGIN = MBED_APP_START, LENGTH = MBED_APP_SIZE
   SRAM2 (rwx)  : ORIGIN = 0x10000188, LENGTH = 32k - 0x188
   SRAM1 (rwx)  : ORIGIN = 0x20000000, LENGTH = 96k
 }